1. Field of the Invention
This invention relates to monitoring systems and particularly those systems particularly adapted to monitor the blood pressure of a patient.
2. Description of the Prior Art
There are many applications in which it is desired to provide a portable system capable of measuring the blood pressure of a patient. For example, when a patient is coupled to an aritificial kidney, a patient may lose approximately two to three quarts of water from his vascular system. If too much water is lost, the patient's blood pressure may drop rapidly, thereby endangering the patient's life. Therefore, it is necessary to closely monitor the patient's blood pressure during such treatment so that appropriate action may be taken if his blood pressure unduly varies.
In FIG. 1, there is shown a blood pressure waveform to be measured by the blood pressure monitoring system of this invention. As the heart muscle pumps blood through the vascular system, the blood pressure varies approximately in a manner as shown in FIG. 1, having maximum points indicative of systolic blood pressure and minimum points indicative of diastolic blood pressure. Systolic blood pressure may be defined, for the purposes of this application, as the peak force per unit area with which the blood is pushing against the artery walls when the ventricles of the heart are contracting, i.e. Point A of the waveform of FIG. 1. Diastolic blood pressure is the minimum pressure exerted by the blood upon the artery walls when the ventricles are relaxed, i.e. Point B as shown in FIG. 1. The mean blood pressure "M" as labeled in FIG. 1, is defined as the average pressure of the blood pressure waveform and is defined in accordance with the following equation: ##EQU1## where p(t) is the blood pressure as a function of time, and T is the period of cyclical waveform as shown in FIG. 1.
There are certain factors that affect blood pressure waveforms. Directly, arterial blood pressure is influenced by the blood volume within the arteries, which in turn is a function of the cardiac output and peripheral resistance. Therefore, any change in cardiac output or peripheral resistance may cause a change in the blood pressure waveform. Further, the cardiac output is equal to the stroke volume times the heart rate. As a result, any factor that tends to change the heart rate or stroke volume, tends to change the cardiac output. For example, an increase in cardiac output may result in a corresponding increase in blood pressure providing peripheral resistance stays constant. Peripheral resistance is defined as the fluidic resistance to blood flow. Any factor that increases the peripheral resistance, will result in an increase in the arterial blood pressure, assuming the cardiac output remains constant. The increase or decrease of the systolic, diastolic or mean blood pressure, and the amounts thereof, are indicative of designated mechanisms within the vascular system, thus indicating the status of the circulatory system.